List the date of sale as 12/31/2020 - you donated ("gave up") the vehicle in 2020 and that is the tax year in which you can claim the charitable deduction.
This is a very common occurrence. Contributions of this nature (car, boats, etc) are often made later in the tax year and the value of the donation is not established until early in the following year. The fact that the charity didn't sell the item (i.e. establish the value) until 2021 does not change the economic fact that you made the donation in 2020 and that it did have a measurable value. You just didn't find out that value until 2021.
